A radioactive ray generating apparatus, which is generally usable as a radiation source, includes an electron emission source that can emit electrons and a radioactive ray generation mechanism that causes generated electrons to collide against a target, which is made of a material having a larger atomic number (e.g., tungsten), to generate radioactive rays. The radioactive ray generated from the target propagates in all directions. Therefore, a shielding member is provided to shield unnecessary radioactive rays that are not available for an image capturing operation. However, if a radioactive ray generating apparatus is configured to include a radioactive ray tube surrounded by a shielding member, downsizing the radioactive ray generating apparatus is difficult.
As a conventional method capable of downsizing the radioactive ray generating apparatus, it is useful to configure the radioactive ray generating apparatus so as to include a transmission-type target because an amount of a shielding material (e.g., lead) to be used to shield unnecessary radioactive rays can be reduced. For example, Japanese Patent Application Laid-Open No. 2007-265981 discusses a structure in which a second shielding member (i.e., a back shielding member) and a first shielding member (i.e., a front shielding member) are provided on both sides of a transmission-type target. According to the structure discussed in Japanese Patent Application Laid-Open No. 2007-265981, an electron beam passes through an aperture of the second shielding member and collides against the target to generate radioactive rays that travel in all directions. The second shielding member can shield radioactive rays emitted toward an electron emission source from the target. Among the radioactive rays which are generated from the target and travel in a direction opposite to the direction of the electron emission source, radioactive rays to be used in an image capturing operation can be extracted from an aperture of the first shielding member. The first shielding member can shield unnecessary radioactive rays. The first and second shielding members are functionally operable as a means capable of releasing heat generated from the target.
Further, as another conventional method capable of downsizing the radioactive ray generating apparatus, it is useful to increase efficiency in generation of radioactive rays to extract an intended amount of radioactive rays with a lesser amount of electric currents. In this method, it is conventionally known that approximately one-half of electrons that have reached a target become reflection electrons and do not contribute to generation of radioactive rays. Therefore, a method capable of effectively reusing the reflection electrons is conventionally discussed. On the other hand, it is conventionally known that reflection electrons may induce generation of radioactive rays from a portion other than a focal point and may electrify constituent components of a radioactive ray tube. To solve the above described issue and improve the efficiency in generation of radioactive rays, a conventional technique discussed in Japanese Patent Application Laid-Open No. H9-171788 uses an electron reflection member that has a channel configured to form an aperture whose diameter decreases with increasing distance from an electron emission source to guide reflection electrons toward a transmission-type target.